1. Field of the Invention
The present invention relates to a novel material for electron transportation and emitting layers, and an organic electroluminescent display using the same.
2. Description of the Related Art
An electroluminescent display is a luminescent device using electroluminescence of a solid fluorescent material. Current practical technology uses an inorganic electroluminescent display with an inorganic material as an illuminant. However, the conventional inorganic electroluminescent display requires a high voltage of 100 V or more for luminescence, and it is difficult for it to emit blue light. As a result, full colorization by the three colors of RGB (red, green, blue) is difficult.
Although studies of electroluminescent devices using organic materials have attracted attention for a long time, only a small number of devices have been commercialized due to their lack of stability and low efficiency compared to conventional display devices such as liquid crystal displays and cathode ray tubes. Organic electroluminescent displays are based on the theory that electrons and holes injected into an organic thin film of small molecules (sublimable molecules) or a polymer by way of an anode and a cathode form an exciton, and light with a specific wavelength is generated when the high-energy exciton returns to its ground states. This effect was first discovered with a single crystal of anthracene by Pope, et al. in 1965 (M. Pope et al., J. Chem. Phys., 42, 2540, 1965). In 1987, an organic electroluminescent display with a laminated structure of a function-separation type dividing organic material into a hole transporting layer and a emitting layer was suggested by Tang, from Kodak Company, and it has been confirmed that low voltage of 10 V or less and high luminance can be obtained (Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913). Since then, organic electroluminescent displays began to attract attention. At present, studies of organic electroluminescent displays with this same function-separation type laminated structure are progressing.
The structure of a conventional organic electroluminescent display contains, as shown in FIG. 1, a substrate (1), an anode (2), a hole injection layer (3) for receiving holes from the anode, a hole transportation layer (4) for transporting holes, an emitting layer (5) in which holes and electrons are bound to emit light, an electron transportation layer (6) for receiving electrons from a cathode to transfer them to the emitting layer, and a cathode (7). The emitting layer (5) can be composed of two or more different molecules to further increase the device efficiency by separating the roles of light emission and transporting both electrons and holes. Generally, the molecule transporting both holes and electrons is called a host molecule and the other molecule emitting light is called a dopant molecule. Usually, the emitting layer (5) is composed of a majority of host molecules and small amounts (1 to 20%) of dopant molecules. The requirements of a dopant molecule include high fluorescent or phosphorescent efficiency with proper band structure relative to the host molecule. According to the circumstances, a small amount of fluorescent or phosphorescent dye is doped on the electron transportation layer (6) or on the hole transportation layer (4) to comprise an emitting layer therein without a separate emitting layer (5). Also it is possible to dope more than one layer such as electron transporting, emitting and hole transporting layers to improve the operational stability or to have multiple emissions. Organic thin films between two electrodes are formed by vacuum deposition, spin coating, ink jet printing, roll coating, etc. For efficient injection of electrons from the cathode, a separate electron injection layer is often inserted.
The reason for manufacturing an organic electroluminescent display with a multi-layered thin film structure includes stabilization of the interfaces between the electrodes and the organic layers. In addition, in organic materials, the mobility of electrons and holes significantly differ, and thus, if appropriate hole transportation and electron transportation layers are used, holes and electrons can be efficiently transferred to the luminescent layer. Also, if the density of the holes and electrons are balanced in the emitting layer, luminescence efficiency can increase.
The proper combination of organic layers described above can enhance the device efficiency and lifetime. However, it has been very difficult to find an organic material that satisfies all the requirements for use in practical devices. For example, tris-(8-hydroxyquinoline) aluminum (Alq3) has been used as an electron transport material for more than 15 years, and there have been many publications and patents claiming they have superior properties. Therefore, it is crucial to find a molecule that has superior properties compared to the conventional material in all practical aspects, such as high efficiency, thermal stability, operational stability and maintaining the driving voltage before and after operation.